In the magnetic recording method, the magnetic bit with higher recording density is more affected by the ambient temperature and such. To address this problem, a recording medium having greater coercive force is required. Use of such a recording medium requires a greater magnetic field for recording. The upper limit of the magnetic field produced by the recording head depends on the saturation flux density, and the value of the saturation flux density is so close to the limit of the material that a drastic increase cannot be expected. To solve this problem proposed is the following method: Causing magnetic weakening by local heating for recording, recording while the coercive force is reduced, and stopping the heating to facilitate natural cooling, whereby the recorded magnetic bit is stabilized. This is referred to as a thermally assisted magnetic recording method.
In the thermally assisted magnetic recording method, a recording medium is preferably heated instantaneously, and the heating mechanism must not to be contacted to the recording medium. Therefore, heating is generally performed by absorption of light. The method of using light for heating is called the optically assisted method. A minute light spot having a size smaller than a wavelength of the used light is required when the optically assisted method is used for high-density recording.
Therefore, utilized is an optical head that employs the near-field light produced at an optical opening having a size smaller than the wavelength of the incident light (Specification of U.S. Pat. No. 6,944,112).
The optical recording head disclosed in the Specification of the U.S. Pat. No. 6,944,112 includes a writing magnetic pole, and a plane waveguide containing a core layer and a clad layer adjacent to the writing magnetic pole. The core layer is provided with at least one parabolic edge that reflects an electromagnetic wave inside the core layer and leads it to the focal point. The apex portion, where the focal point of the parabola is located, has a flat surface shape similar to the shape made by cutting off a tip of a parabola. This apex portion is provided close to the air bearing surface (ABS), where the recording head is opposed to a recording medium.
The core layer is provided with a diffraction grating that leads light into the core layer. For example, when the diffraction grating is radiated with a laser beam, the laser beam is introduced into the core layer and converged at the focal point located on the apex portion. The recording medium is radiated with the light emitted from the apex portion and thereby heated.
Although in the optical recording head disclosed in the Specification of the U.S. Pat. No. 6,944,112, the laser beam applied to the diffraction grating is led into the core layer of a plane waveguide and is guided toward the apex portion, the light directly coming to the plane surface having a shape formed by cutting off the tip of the parabola does not converge at the focal point. Thus, there is a problem that the laser beam having a Gaussian distribution of intensity of the light applied to the diffraction grating does not effectively converge at the focal point in some cases, and light cannot be effectively emitted from the apex portion of the core.
In view of the prior art problems described above, it is an object of the present invention to provide an optical device where the light applied to a light introducing section can be efficiently emitted from the apex portion of a core layer, an optical recording head containing this optical device, and an optical recording apparatus including this optical recording head.